Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising.
Some transmissions have a collection of gearing and shift elements configured such that engaging various subsets of the shift elements establish various power flow paths between an input shaft and an output shaft. These various power flow paths are commonly referred to as gears even when they are not associated with a particular physical gearing element in the transmission. These various power flow paths establish different speed ratios between an input shaft and an output shaft. Automotive transmission typically provide multiple forward power flow paths with positive speed ratios and at least one reverse power flow path with a negative speed ratio. To change from one speed ratio to another speed ratio, one of more shift elements are disengaged and one or more shift elements are engaged in order to change which power flow path is utilized. A continuously variable transmission may implement a number of virtual gears to simulate operation of a step ratio transmission.